Polysilicon has hitherto been used in electrodes or conductors of LSI semiconductor devices. Accompanied with the tendency toward higher integration of LSI semiconductor devices, silicides of molybdenum and tungsten have been widely utilized. Increasing attention is now paid to the practical use of the silicides of titanium and cobalt. Also, trials are progressing using cobalt as conductors or wirings instead of the Al or Al alloys hitherto used. The electrodes or conductors or wirings of cobalt or cobalt silicide are typically formed by sputtering a cobalt target in an argon atmosphere.
It is important to note that semiconductor members formed after sputtering include metal impurities detrimental to semiconductor devices in minimum quantities. That is, it is important that the following are minimized: (1) alkali metals such as Na and K, (2) radioactive elements such as U and Th and (3) transition metals such as Fe, Ni and Cr.
The alkali metals such as Na and K easily move in gate insulation films and may cause the deterioration of MOS-LSI interface characteristics. And, radioactive elements such as U and Th may cause soft errors in device elements owing to alpha rays emitted from these radioactive elements. On the other hand, heavy metals such as Fe, Ni and Cr also may cause problems in connections at the interface.
Commercially available cobalt, so-called crude cobalt lump, contains tens of ppm Fe and several hundred ppm Ni as impurities. Electrolytic refining is first considered for producing high purity cobalt from this crude material. However, since the standard electrode potentials of Ni and Fe impurities on the one hand and Co on the other hand are very near to each other, high purification is difficult to achieve with normal electrolytic refining operation.
For purification by electrolytic refining, it is necessary to remove the impurities in the electrolyte by a solvent extraction process, while maintaining the concentrations of Ni and Fe in the electrolyte to no more than 1.3 mg/l on average and no more than 0.1 mg/l on average, respectively, in the case where the Co concentration in the electrolyte is 40 to 60 g/l. This necessitates very strict control. Further, removal of Ni by solvent extraction requires a special solvent such as ALKYLOXYM. The operation is complex since cobalt is co-extracted. Further, there is a problem that the extracting solvent dissolves into the electrolyte to incur a loss of solvent.
In addition, as another problem, there is sometimes found the phenomenon that electro-deposited cobalt peels off from a cathode sheet. It has also been pointed out that there are cases where it is difficult to lower the gas component, inclusive of gaseous components and gasifiable components such as carbon, oxygen and others of electrodeposited cobalt. Gas components such as carbon and oxygen are considered to be undesirable since when cobalt is used for sputtering target applications, the gas component gives rise to particle generation.